Foods such as vegetables and fruits are largely divided into non-heated foods (raw foods) and heated foods. Very few in between these two states are produced. Non-heated foods are further divided into ones whose cells are alive (edible) and perishable plants such as salad and fresh fruit) and the others whose cells are dead (pickles, that is tsukemono, and vinegar pickles). Some foods have toxicity, bitterness, astringency, harshness, grassiness and the like to protect themselves against animals, insects and the like, under the condition that their cells are alive. On the other hand, when their cells die, edible plants are swiftly resolved by autolytic enzymes and so become easy to eat and digest.
Numerous microorganisms are attached to a surface of edible and perishable plants. Usually, even after being washed, the plants retain microorganisms in which the number of bacteria is between 104 and 106 per gram. As there have frequently occurred food-poisoning in recent years, the foods are commonly washed in a germicidal agent such as sodium hypochlorite solution. But a complete sterilization is difficult, and rinsing the foods after the above treatment to remove the germicidal agent largely decreases their quality. As a result, many schools have given up providing their pupils with raw vegetables (salad) for lunch.
On the other hand, in the case of pickles and vinegar pickles of a non-heated type and whose cells are dead, a method is adopted in which microorganisms are controlled from reproducing while their cells are being killed by an osmotic pressure of salt, sterilizing power of vinegar and the like. In this case, however, the low temperature and the salt restrain the resolution of the cells by autolytic enzymes, so that it takes a long processing time, which makes the pickles and the vinegar pickles much too salty and sour. Pickles using highly concentrated salt to preserve them from rotting should be exposed to water so as to remove the salt. Accordingly the tastiness factor and nutritive elements are greatly damaged, and the real situation is that the use of additives helps the pickles stay tasty for longer days. Furthermore, as a substantial amount of water is used (usually 50 tons of water is used to make one ton of pickles), waste water disposal becomes very costly.
In the case of heating edible plants, their tissue softens, and the factors such as harshness, grassiness and the like disappear. But the rapid heating makes the cellular resolution by autolytic enzymes insufficient, the foods lose their original taste, and nutritive elements (especially Vitamin C) are greatly damaged.
The technologies of processing edible and perishable plants by heating at a medium temperature include a method of processing vegetables in a warm aqueous solution of a medium temperature range (between 40° C. and 70° C.) (Japanese Patent Laid-open Publication No. 5-68505), one of making pickles by steaming them (Japanese Patent Laid-open Publication No. 6-169690), one of sterilizing and preparing foods by steaming them (Japanese Patent Laid-open Publication No. 11-155513), one of pickling Japanese apricots with salt while maintaining a high atmospheric temperature (Japanese Patent Laid-open Publication No. 9-65824), one of making pickles in a highly concentrated saline solution of a medium temperature (Japanese Patent Laid-open Publication No. 8-289722), one of washing vegetables in a warm aqueous solution of between 35° C. and 55° C. (Japanese Patent Laid-open Publication No. 11-18740), and so on. These methods, however, do not fix the temperature range from the viewpoint of cellular resolution by autolytic enzymes, so that the foods have neither tastiness nor sweetness.
Since edible and perishable plants are alive, nutritive substances of the tissue are lost by metabolism as time passes, so that the plants wither, become fibrous and less tasty. In due time, they develop mold from the parts where the cells died from such as cuts, and rot all over. Perishable vegetables keep only for a week or so at the longest. Perishable fruits keep longer than vegetables but about one month is the limit.
Generally, perishable plant foods have hard cell walls, lack flexibility and contain a lot of water (80-95% of them is water) as compared with animal foods. Therefore, their cells are greatly destroyed by freezing and a large amount of exudate, i.e. “drip”, flows out when defrosting. In addition, browning, softening and rotting are caused by enzymes. Accordingly, it is said that perishable plant foods are not suitable for freezing.
On the other hand, frozen processed vegetables and fruits are commercially available. In this case, vegetables and fruits are cut into small pieces and blanched, that is to say, dipped into hot salt water of a high temperature for a short time (for example, 90° C.×10 seconds-1 minute) by way of treatment before freezing. This heating treatment softens the tissue, deactivates the enzymes and fixes chlorophyll, which prevents the products from becoming low in quality while they are kept frozen as well as when they are defrosted.
Since blanching means cooking by heating, it is inevitable that both texture and nutritive substances of perishable foods are lost. Furthermore, the shortness of heating time makes the equal-treatment difficult, resulting in variation in quality. On top of it, without cellular resolution by autolytic enzymes, harshness and grassiness remain, and the original taste of the foods is lost.
Since non-heated processed foods, for example, pickles, whose cells are dead, have been dehydrated with salt, and the cell walls have been destroyed, the foods are hardly damaged by freezing. It has already come into practice to store heavily salted pickles such as nozawana and takana, whose tissues are comparatively solid, in a freezer.
The cells of edible and perishable plants are alive. If the plants are left as they are, they show signs of nutritive deterioration and change in taste, which decreases their commercial value. Considering the above, edible and perishable plants should be cooked as soon as possible or stored in a freezer. When the cells die, the plants begin to be resolved by autolytic enzymes, but at the same time they are attacked by the microorganisms attached to them and rotting progresses.
An object of the present invention is to offer a food processing method for edible and perishable plants, which kills both the cells and the microorganisms of them, as well as causes the cellular resolution by autolytic enzymes, and accordingly changes undesirable components for foods (harshness, bitterness, astringency, grassiness and the like) into tastiness, sweetness, sourness and flavors.
A further object of the present invention is to offer a food processing method for edible and perishable plants which is excellent for storing them in a freezer, in addition to the above advantages.